1. Field of the Invention
The present invention relates to a radiation detector which uses a scintillator structure having an optical anisotropy.
2. Description of the Related Art
In an image detector for radiodiagnosis, an image of X-ray radiography is obtained as a digital signal by detecting an irradiated X-ray. The radiation detectors are roughly classified into a direct X-ray detector and an indirect X-ray detector. The indirect X-ray detector is a detector which changes X-ray into visible light by using phosphor, and converts the visible light into a charge signal by a photoelectric transducer so as to obtain an image. The detectors are arranged in a two-dimensional array. In a high-definition radiation detector, the pixel size needs to be reduced, and in a radiation detector used for obtaining a high-definition image as described above, a crosstalk of light can be suppressed by using a scintillator structure having an optical anisotropy.
In a high-definition radiation detection device, the pixel size is small, and hence, when a scintillator structure having an optical anisotropy is used, there has been newly found a problem that an influence on an image degradation by a flaw generated in the structure is not negligible. Conventionally, when the flaw size is sufficiently small as compared to an area of a light receiving portion, a loss influence of an optical waveguiding is negligible. However, as an area of a light receiving portion per pixel decreases, the loss influence of the optical waveguiding increases so that the influence on image degradation increases. This is because a flaw may be generated in the above-mentioned structure due to cutting, polishing, or handling regardless of the size of the pixel. The above-mentioned loss influence is caused by a material, typically the air, which has a refractive index different from that of the structure, at the portion where the flatness has been lost by the flaw.
When “X” represents an allowable threshold value of received light intensity fluctuation among the pixels, “A” represents a propagation loss coefficient per unit area by a flaw, “S0” represents an area of the light receiving surface per pixel, and “S” represents an area of a flaw per one pixel, the following expression needs to be satisfied in order to keep the received light intensity fluctuation among the pixels equal to or less than “X”.A×S/S0≦X  Expression 1
Here, the propagation loss coefficient A per unit area by a flaw is 0≦A≦1, which is 0 when there is no flaw and is 1 when the light guiding is completely blocked by a flaw. In a strict sense, the propagation loss coefficient A per unit area by a flaw relates to a depth and a shape of the flaw. For simplification, it is assumed here that the flaws have the same depth and shape. Specifically, it is assumed that the depth and width of the flaws are the same, but the length or the number of the flaws is different. Such flaws correspond to, for example, flaws generated by polishing marks due to foreign matters having the same size, and the area S of the flaw per pixel as described above is determined by the length of the flaw or the number of the flaws.
The area S0 of the light receiving surface per pixel and the area S of the flaw per pixel are restricted, and hence, in order to decrease the received light intensity fluctuation among the pixels, i.e., the value of the left side of the expression 1, the propagation loss coefficient A per unit area by the generated flaw inevitably needs to be decreased.
As the scintillator structure having the optical anisotropy described above, a scintillator structure having a phase-separated structure can be used. The scintillator structure having the phase-separated structure includes a first principal plane and a second principal plane which are positioned as different planes, and includes two phases including a first phase having a unidirectionality in a direction between the principal planes, and a second phase which fills a side of the first phase. A material of a higher refractive index phase of the two phases functions as a scintillator, thereby having an optical waveguiding property. In the radiation detection device using the scintillator structure having the phase-separated structure, there has been newly found a problem in that, when the flaw is positioned in the first or second principal plane of the structure and the minimum size of the depth or length is ¼ or more of an average distance “d” between proximate first layers of the above-mentioned phase-separated structure, the influence on the image degradation becomes remarkable. This depends on the number of the first layers per pixel, which is characterized by the above-mentioned average distance “d”, with respect to the area S0 of the light receiving surface per pixel. As the number of the first layers per pixel decreases, the influence on the image degradation increases.